Method of making electric heating elements



April 15, 1969 w. J. slMs 3,438,128

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METHOD OFMAKING ELECTRIC HEATING ELEMENTS Filed Feb. 1, 1966 sheet 2 of s FIG.7.

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April 15, 1969 w. J. slMs METHOD OF MAKING ELECTRIC HEATING ELEMENTS Filed Feb. 1, 1966 5' ofs Sheet FIG. lOd

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United Statesg Patent O 3,438,128 METHOD OF MAKING ELECTRIC HEATING ELEMENTS William John Sims, Buxted, near Uckfield, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Feb. 1, 1966, Ser. No. 524,064 Claims priority, application Great Britain, Feb. 4, 1965, 4,999/ 65 Int. Cl. H01c 15/04, 17/00 U.S. Cl. 29-615 b 3 Claims This invention relates to tubular sheathed electric heating elements that is to say in which electrical resistance wire is surrounded by a tubular sheath and insulated therefrom by electrically insulating but heat conducting material, such as for example compacted granulated magnesia, and the invention is especially, but not exclusively, applicable to tubular sheathed heating elements for use as boiling plate or grll, elements for electric cookers.

It has been proposed to increase the available thermal output of a tubular sheathed electric heating element having a sheath of circular cross-sectional form by enclosing two helically wound electrical resistance wire elements side by side within the circular sheath. Such an arrangement however suffers from the disadvantage that the space taken up by the cores of the helically coiled wire elements is wasted.

An object of the invention is to provide a tubular sheathed electric heating element which is of Compact form an-d high heat output per unit length.

According to the present invention there is provided a tubular sheathed electric heating element comprising a tubular sheath of generally oval cross sectional form, at least two electrical resistance wire elements of flat zigzag form arranged in spaced relation, preferably one above the other or side by side, within the sheath and electrically insulating but heat conducting material electrically insulating the electrical resistance wire elements from the sheath and one another, at least one of the sheaths broader sides preferably being flattened.

By using at least two electrical resistance wire elements of flat zig-zag form arranged one above the other or side by side in this fashion, optimum use can be made of their potential heat output without detracting from minimum insulaton thickness requirements both between the electrical resistance wire elements and between said elements and the sheath.

Furthermore, full use may be made of the electrical resistance wire elements for providing maximum fleXibility of heat output and control, for example to provide an initially rapid heating up, i.e., boosting, period.

One such possible initial boost arrangement is by series/ parallel switching, i.e., initially the two electrical resistance wire elements are connected in parallel to give maximum heat output and then after a predetermined time or after a particular temperature has been reached the electrical resistance wire elements are automatcally Switched to a series connection, thus decreasing the output to a quarter of its original value.

Alternatively one electrical resistance wire element may serve as a main or running wire element, while the other can act as a boost element; the electrical resistance wire itself and the displacement of the electrical resistance wire elements being such as to provide an initially rapid heating up period, followed by a re'duction in thermal output as the required temperature is reached. It can be arranged for the main or running wire element to be located adjacent a heating surface of the sheath so as to provide a higher temperature there.

One of the two electrical resistance wire elements may have a relatively high temperature coeflicient of resistance and the other a relatively low coeflicient. The low temperature coeflicient of resistance wire element may serve as the booster element by connection of the wire elements in series, the initial boost being obtained because the cold low resistance of the high temperature coefficient of resistance wire element results in a relatively high initial current fioW through the other wire element, the temperature of which therefore rises rapidly. The high temperature coefficient of resistance wire element may serve as the booster element by parallel connection, of the wire elements, the initial -boost being obtained because in its cold low resistance state the high temperature coeflicient of resistance wire draws a relatively high current. In each case, loading is automatically vreduced to a steady state, i.e., running condition when a predetermined temperature is reached.

The invention also consists in a process for producing a tubular sheathed electric heating element including the step of preforming as by rolling, a generally oval sheath into a form which will accept at least two flat zig-zag electrical resistance wire elements in a particular spaced relationship and maintain this relationship during an initial stage of compaction as by pressing.

The tubular sheathed electrc heating element according to the invention may be formed conveniently into a substantially flat spiral formation with the broader surfaces arranged in the plane of the spiral, enabling the element to be used as a boiling plate for heating liquids, or with the broader surfaces substantially perpendicular to the plane of the spiral, for example, as an air heater.

One convenient arrangement of electric heating element of the tubular sheathed type and the process of produeing the same according to the invention will now be described by way of example with reference to the accompanying drawing, in which:

FIGS. 1, 2, 3 and 4 are cross sections to an enlarged scale of the tubular sheathed heating element in Successive stages of manufacture,

FIG. 5 shows the form of the cross section of the completed tubular sheathed heating element,

FIG. 6 shows to a smaller scale the form of flat zigzag electrical resistance wire elements used in the tubular sheathed heating element,

FIG. 7 illustrates in plan an arrangement for forming flat zig-zag electrical resistance wire elements,

FIG. 8` is an end view of two flat zig-zag wire elements with a spacer bar therebetween in a stage in manufacture of the tubular sheathed heating element,

FIG. 9 is an incomplete vertical section on the line VIII-VIII of FIG. 9a of two flat zig-zag elements with their leads in a stage in the manufacture in which the tubular sheath is read-y for filling with magnesia,

FIG. 911 is an end view of the tubular sheathed heating element at the FIG. 9 stage,

FIIG. 10 is an outside view of part of a tubular sheathed heating element after filling with magnesia and flattening i.e. at the stage shown in FIG. 5,

FIG. lOa is an end view of FIG. 10, and

FIG. 11 is a plan of a final spiral form of the tubular sheathed heating element for use as a boiling plate.

The tubular sheathed electrical heating element shown i-n FIGS. 5, 10 and lOa comprises an outer tubular sheath 1 of, for example, the material sold under the trademark Inconel, which is an alloy of approximately 76% nickel, 21% chromium and 6% iron. The sheath 1 contains two electrical resistance wire elements 2 and 3, of, for example, the .material sold under the trademark Nichrome, wh-ch is an alloy of 60% nickel, 24% iron, 16% chromium and 0.1% carbon. The wire elements 2 and 3 are of flat zig-zag form (as shown in FIG. 6) and equally spaced from the sheath 1 and each other. The wire elements 2 and 3 are surrounded by and separated from one another and the sheath 1 by a compacted mass of magnesia 4, which is an electrically insulating but heat conducting material. The wire of the wire elements 2 and 3 is itself also of flattened cross-section to reduce the overall thickness of the fiat wire elements 2 and 3 to increase rigidity for ease of handling.

In manufacturing the tubular sheathed heating element the zig-zag resistaince wire elements 2 and 3 are formed in a continuous fashion e.g., by passing wire round a series of pins 8 as shown in FIG. 7 and a-re then cut to the length required. The wire elements 2 and 3 are flattened by passing them between a pair of contrarotating flat rolls (not shown).

The tubular sheath 1 initially of round section (FIG. 1) is formed into the grooved generally oval shape of (FIG. 2) by a series of rolling Operations, which can be arranged as a continuous process when large scale production is required. After forming the sheath 1 into the oval shape shown in FIG. 2 by passing the tube 1 between rolls, an end of the tube is arranged over a stationary mandrel having grooves Sa and Sb as shown in FIG. 2. The mandrel is held stationary between two identical rolls 9 one of which is shown in FIG. 2, the other is arranged on an opposite side of the tube 1 in FIG. 2 to form the groove 7 (FIG. 4). When the tube is in position the rolls 9 are set in relation to the mandrel 5 so that as the tube is fed over the mandrel the grooves 6 and 7 are formed accurately, especially at tia and lb (FIG. 3) adjacent the grooves 6 and 7, to provide an accurate fit for the wire elements 2 and 3 (see FIG. 3). The form of the mandrel 5 supports the internal surfaces in a manner such that the grooves 6 and 7 are correctly formed.

In an alternative method of arriving at the generally oval tube of FIG. 3 the grooves 6 and 7 may be rolled or pressed in a flat strip of Inconel, which is then formed to a tube of general oval form as in FIG. 3 by rolling and seam welding along its length.

The flat zig-zag wire elements 2 and 3 with leads 11 secured thereto, as will be more fully hereinafter described, are arranged vertically in slight tension and spaced apart by a fiat spacer bar (an end view of which is shown in FIG. 8) extending along the length of the wire elements 2 and 3.

The wire elements 2 and 3 are set up with junctions 11a between the wire elements 2 and 3 and the leads 11 staggered, as shown in FIGS. 9 and 9a so as to provide maximum clearance between them. The sheath LI, which is of the generally oval form shown in FIG. 3 is slid over the assembly of elements 2 and 3 and spacer bar 10. The wire elements 2 and 3 are now located by the internal profile of the sheath 1 at la and lb as shown in FIG. 3. The fiat spacer bar 10 is now withdrawn and the lower end of the sheath 1 is sealed by a bush 12 of a fusible ceramic material to prevent escape of magnesia during a filling Operation, the bush l12 being provided with holes to allow the leads 11 to pass therethrough.

The tube is machine filled with magnesia from a storage hopper through a flexible tube connected to the top end of the tube 1. The tube 1 is vibrated during ffilling to ensure optimum compaction of the magnesia. After filling the upper end of the tube is sealed with a bush similar to 12 shown in FIG. 9 and the filled tubular element is removed -from the machine.

The magnesia 4 is then compressed to a degree, which will prevent movement of the wire elements 2 and 3. The corrugated sides of the generally oval sheath 1 (FIG. 3) are held in a die 13 (FIG. 4) whilst pressure P is applied from a punch '14 to form the sheath 1 to the generally square section shown in full line in FIG. 4. A further pressing without restraint on the corrugated sides of the sheath forms the tubular sheathed element to substantially the cross sectional form shown in FIG. 5, i.e., generally rectangular. The magnesia 4 is thereby further compacted to a density required for optimum thermal conductivity and electrical insulation and at the same time caused to fiow between the side edges of the fiat zig-zag elements 2 and 3 a-nd the sheath 1. The initial compression before the restraint is removed from the corrugated sides of the sheath 1 is such that the location of the zig-zag wire elements 2 and 3 is taken over by the magnesia during the final stage, lduring which the corrugations are forced out of the sheath 1 to reach the FIG. 5 shape. The leads shift in pa'ssing from FIG. 3 to FIG. 5 to the position shown in FIG. 10a.

As an alternative to the successive pressing Operations just described the tubular sheathed element could for larger scale production be passed between suitably formed rolls.

The tubular sheathed element is next annealed during which the bushes 12 at each end melt to form moisturetight seals.

Prior to a final coiling Operation into the spiral configuration shown in FIG. 11, one end l215 of the tubular sheathed element is bent at right angles to the remainder of the sheathed element so as to enable that end to be subsequently further formed to pass under the spiral and in conjunction with the other end of the element 1'6 to be fitted into a terminal housing 14 (FIG. 11).

The procedure for securing the leads 11 to the wire elements 2 and 3 referred to above is by twisting the wire back on itself (FIG. 9) and then butt welding at 11a the twisted ends of the wires as of wire 2, to the ends of the leads 11. The leads 11 are of different lengths as shown in FIG. 9 to provide the staggering referred to above. The twisting provides a more rigid junction. The leads may for a low resistance connection between the supply and the resistance wire elements 2 and 3 be of Monel metal with copper inserts therein. The inner ends of the leads 11 in the sheath may also be of recessed form for receiving the twisted ends of the wire.

Although the process of manufacture has only been described for a tubular sheathed element with two zig-zag wire resistances mounted one above the other, which would be the more common case, such a process could be applied to a wide variety of element lengths, cross sections, materials and applications. The sheathed element according to the invention is primarily, but not exclusively, intended for use as a boiling plate and for this purpose the size of cross-section and hence its mass is kept to a minimum so as to reduce the thermal inertia in operaton, thus providing rapid heating up.

Inconel is obtainable in the United States from The International Nickel Co., W.Va., and Nichrome is available in the -United States from Driver Harris Co., Harrison, NJ.

What I claim is:

1. The method of manufacturing a tubular sheathed electric heating element comprising a tubular sheath of generally oval cross sectional form, at least two electrical resistance wire elements of flat zig-zag form arranged in spaced relation within the sheath and electrically insulating but heat conducting material electrically insulating the electrical resistance wire elements from the sheath and one another including the steps of forming a tubular sheath of generally oval cross sectional form with opposed longitudinal depressions in its opposite sides, filling the sheath with electrical conducting but heat insulating material with the wire elements held in spaced substantially parallel relationship by the depressions, subjecting the sheath to an initial stage of compaction with pressure applied to the depressions so that they retain the wire elements in position and to a subsequent stage of compaction without pressure applied to the depressions whereby the depressions are forced out of the sheath and the insulating material flows between side edges of the wire elements and the sheath.

2. The method of manufacturing a tubular sheathed heating element as claimed in claim 1, in which the electrical resistance wire elements are held in spaced relation by a spacer means while the Wire elements are located in the sheath, the spacer means being withdrawn prior to filling the tube with the electrical insulating but heat conducting material.

3. The method of manufacturing a tubular sheathed heating element as claimed in claim 2, in which the one end of the sheath is closed with a fusible cerarnic plug prior to filling the tube with the electrical insulating but heat Conducting material.

1,669,385 5/1928 Wiegand et al 29-614 6 1,960,221 5/1934 Kelly et al. 29-614 2,360,265 10/ 1944 Osterheld 338-293 X FOREIGN PATENTS 528,718 11/1940 Great Britain.

.TO-HN F. CAMPBELL, Primary Examiner.

J. L. CLI-NE, Assistant Examz'ner.

U.S. Cl. X.R. 

1. THE METHOD OF MANUFACTURING A TUBULAR SHEATHED ELECTRIC HEATING ELEMENT COMPRISING A TUBULAR SHEATH OF GENERALLY OVAL CROSS SECTIONAL FORM, AT LEAST TWO ELECTRICAL RESISTANCE WIRE ELEMENTS OF FLAT ZIG-ZAG FORM ARRANGED IN SPACED RELATION WITHIN THE SHEATH AND ELECTRICALLY INSULATING BUT HEAT CONDUCTING MATERIAL ELECTRICALLY INSULATING THE ELECTRICAL RESISTANCE WIRE ELEMENTS FROM THE SHEATH AND ONE ANOTHER INCLUDING THE STEPS OF FORMING A TUBULAR SHEATH OF GENERALLY OVAL CROSS SECTIONAL FORM AND OPPED LONGITUDINAL DEPRESSIONS IN ITS OPOSITE SIDES, FILLING THE SHEATH WITH ELECTRICAL CONDUCTING BUT HEAT INSULATING MATERIAL WITH THE WIRE ELEMENTS HELD IN SPACED SUBSTANTIALLY PARALLEL RELATIONSHIP BY THE DEPRESSIONS, SUBJECTING THE SHEATH OF AN INITIAL STAGE OF COMPACTION WITH PRESSURE APPLIED TO THE DEPRESSIONS SO THAT THEY RETAIN THE WIRE ELEMENTS IN POSITION AND TO A SUBSEQUENT STAGE OF COMPACTION WITHOUT PRESSURE APPLIED TO THE DEPRESSIONS WHEREBY THE DEPRESSIONS ARE FORCED OUT OF THE SHEATH AND THE INSULATING MATERIAL FLOWS BETWEEN SIDE EDGES OF THE WIRE ELEMENTS AND THE SHEATH. 